Is Feynman's QFT Unitarity Issue Resolved?

In summary, Richard Feynman discusses his work on conventional electrodynamics with retarded interaction and his half-advanced and half-retarded theory. He mentions using a function f with width a2 to calculate finite results for problems, but this led to a violation of unitarity. However, this difficulty has been resolved through modern renormalized perturbation theory.
  • #1
PatrickPowers
240
1
From Richard Feynman's Nobel Prize Speech.

It must be clearly understood that in all this work, I was representing the conventional electrodynamics with retarded interaction, and not my half-advanced and half-retarded theory corresponding to (1). I merely use (1) to guess at forms. And, one of the forms I guessed at corresponded to changing d [delta function] to a function f of width a2, so that I could calculate finite results for all of the problems. This brings me to the second thing that was missing when I published the paper, an unresolved difficulty. With d replaced by f the calculations would give results which were not "unitary", that is, for which the sum of the probabilities of all alternatives was not unity. The deviation from unity was very small, in practice, if a was very small. In the limit that I took a very tiny, it might not make any difference. And, so the process of the renormalization could be made, you could calculate everything in terms of the experimental mass and then take the limit and the apparent difficulty that the unitary is violated temporarily seems to disappear. I was unable to demonstrate that, as a matter of fact, it does.​

Does anyone know whether this difficulty has been resolved?
 
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  • #2
PatrickPowers said:
Does anyone know whether this difficulty has been resolved?
Yes, a long time ago. Modern renormalised perturbation theory has been shown to
preserve unitarity.
 

FAQ: Is Feynman's QFT Unitarity Issue Resolved?

What is QFT and why is it difficult?

Quantum field theory (QFT) is a theoretical framework that combines quantum mechanics and special relativity to describe the behavior of particles at a subatomic level. It is considered difficult because it involves complex mathematical calculations and requires a deep understanding of both quantum mechanics and special relativity.

Has the difficulty of QFT been resolved?

No, the difficulty of QFT has not been completely resolved. While significant progress has been made in developing the theory and making predictions that have been confirmed by experiments, there are still many unanswered questions and challenges in fully understanding and applying QFT.

3. What are some of the challenges in QFT?

Some of the challenges in QFT include renormalization, which involves removing infinities from the mathematical equations, and the relationship between quantum mechanics and general relativity. There are also ongoing debates and research on various aspects of the theory, such as the existence of particles and the role of causality.

4. Are there any recent developments in QFT?

Yes, there have been many recent developments in QFT. Some notable examples include the use of QFT in understanding and predicting the behavior of materials and condensed matter systems, as well as the study of quantum entanglement and its potential applications in quantum computing.

5. Can the difficulty of QFT be overcome?

While it is unlikely that the difficulty of QFT can be completely overcome, ongoing research and advancements in technology and mathematical techniques may lead to a better understanding and application of the theory. It is important to continue exploring and questioning the principles of QFT in order to make progress in our understanding of the universe at a fundamental level.

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